Adding my experience -
I have never before posted in the BOI, though I've used it many times. I'm posting now because this thread caught my attention, and frankly, scared me to death. I also purchased a baby water snake from Underground Reptiles on September 20th, though mine was listed as a CB baby brown water snake. I added it on to a previously placed order consisting of two licorice black rat snakes and a green iguana, and they shipped together on September 21st, for arrival at the local UPS hub on September 22nd where I picked them up. Upon arrival she looked perfectly healthy (her deli cup was marked female, though I haven't verified). As I do with all new arrivals, I immediately placed her in quarantine in a separate room from my established collection.
In her enclosure I used dry repti-bark as a substrate with a half-log hide, a rock, and several other climbing and hiding accessories. I also incorporated a filtered water tub and live feeder guppies, as well as a staple diet of f/t silversides and (occasional) fish-scented f/t mice. I used an overhead heat source to create basking areas, which she used readily. I want to note that I know the dangers of a damp enclosure, so I've gone to great lengths to ensure that hers is always dry.
It was around the end of October that I noticed tiny white blisters showing up on her scales, making her look rough and bumpy. I scoured the internet looking for something that resembled her issue, but everything I read about blister disease and other skin lesions had pictures that looked nothing like her issue. I decided to wait for her to shed, which she did a day or two later. As the OP mentioned, after her shed, she looked perfectly fine again. This being my first water snake, I attributed the rough and bumpy look to a potential shedding issue and continued to monitor her. It happened again before her last shed, and once again, cleared up completely afterward. Now they're coming back.
I didn't think to take photographs when it was at its worst, but I do have pictures from September, October, and the most recent - which was taken on the 16th of this month, and you can see the blisters returning. I also have screenshots of my receipts from Underground.
Adding to all of this, I recently purchased another trio of WC banded water snakes from them on November 14th. The snakes shipped on November 15th and arrived on the 16th, along with three green anoles and three green tree frogs. All snakes shipped in the same bag, and seemed outwardly healthy upon arrival after a brief inspection. One snake in particular didn't seem to be acclimating well at all the next day, and was very 'jittery' - not hiding at all and even biting himself. He passed away on November 18th. The other two are still in quarantine and seem to be doing fine, with no signs of blisters. I do want to make a note that instead of three banded snakes, I was sent two banded and one brown water snake. This was fine with me, however, so no complaints. I also didn't contact Underground after the banded passed away - I chalked it up to potential stress, being wild caught, shipping, etc.
I'm not looking for any kind of restitution or to say anything against Underground Reptiles - all of my other animals from Underground are thriving with no issues whatsoever. I just wanted to chime in here with my own experience and concerns in regard to the blisters and the potential for this to be such a large issue. With all of this new information (to me), I'm going to get all three snakes in to my exotics vet next week after the holiday weekend. If anyone has any questions, I'll do my best to answer them.
I'm attaching pictures of the receipt from Underground for the initial purchase of the CB baby brown water snake, a picture of her the month she arrived, a picture from last month, and a picture from the 16th of this month- where the bumps are becoming visible again.